This paper presents a new coherent modem for use in good quality sound and music broadcasting in the HF band, to be proposed as a candidate to replace actual AM and SSB modulation schemes which are notoriously perturbed by multipaths, Doppler and fading. The main characteristics of this waveform is to be compatible with existing SSB receivers (and so may be a better alternative than satellite broadcasting at higher frequencies), while opening the door for better quality using a large bandwidth compatible with the actual channel frequency spacing. Such a broadcasting scheme may even be implemented using existing power transmitters in a transmission mode named simulcast that allows simultaneous emission of a program in the same frequency channel using an AM compatible SSB analog waveform and the new digital signal. This broadcasting system is proposed as a candidate for international normalization by the NADIB consortium (an European Eureka 1559 project: NArrow Band DIgital Broadcasting). Demonstration of such a modem was done at Montreux and Amsterdam in 1996 and at the NAB in Las Vegas in 1997. Compatibility tests with existing high power transmitters are under way.
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